Recently, composite semipermeable membrane, in which a skin layer including polyamides obtained by interfacial polymerization of polyfunctional aromatic amines and polyfunctional aromatic acid halides is formed on a porous support, have been proposed (Patent document 1). A composite semipermeable membrane, in which a skin layer including a polyamide obtained by interfacial polymerization of a polyfunctional aromatic amine and a polyfunctional alicyclic acid halide is formed on a porous support, has also been proposed (Patent document 2).
The porous supports include, for example, those in which a microporous layer having a substantial separation function is formed on a surface of a substrate. The substrate includes, for example, a woven fabric, a nonwoven fabric, a mesh net and a foaming sintered sheet, made of materials such as polyester, polypropylene, polyethylene, polyamide and the like. In addition, materials for forming the microporous layer include various materials, for example, such as polyaryl ether sulfones (e.g. polysulfones, polyether sulfones), polyimides and polyvinylidene fluorides, and particularly from the viewpoint of chemical, mechanical and thermal stability, polysulfones and polyaryl ether sulfones have been preferably used.
As for these composite semipermeable membranes, chemical resistance that can endure various oxidants, particularly chlorine washing, has been required as a result of pursuit of low cost due to more stable operability and simple usability as well as prolongation of membrane lifetime, in various water treatments including desalination plants.
The composite semipermeable membrane has a practical chemical resistance, but it may not be said that such a membrane has chemical resistance to endure against routine or intermittent chlorine sterilization. Therefore, development of a composite semipermeable membrane having higher chemical resistance as well as a practical level of water permeability and salt-blocking property, particularly development of a porous support exhibiting an excellent chemical resistance, has been desired.
On the other hand, a porous cured epoxy resin that is a separation medium capable of selectively distinguishing a substance having a plane molecular structure, such as dioxin or PCB (polychlorinated biphenyl), and capable of having a low back pressure, as well as able being processed on a large scale, has been developed (Patent document 3). The porous cured epoxy resin is a non-particle aggregation type porous object including a columnar three-dimensional branched structure, wherein the porous object has a porosity of 20 to 80% and an average pore diameter of 0.5 to 50 μm.
The above-mentioned porous cured epoxy resin can be produced by dissolving an epoxy resin and a curing agent in a porogen (a pore-forming agent) to prepare a mixed solution; applying the mixed solution onto a substrate, followed by heating to form a three-dimensional branched structure via a crosslinking reaction; and then removing the porogen. In the case of such a production method, a resin coating film without pores was easy to be formed on the surface of a porous body, and it was necessary to remove the resin coating film for use as a separation medium, thus causing a problem that the production process became complicated.
On the other hand, a method for producing a modified polytetrafluoroethylene film which includes modifying a lumpy molded product of polytetrafluoroethylene powders and cutting the modified product to form a long film has been proposed for the purpose of providing a method capable of producing a modified PTFE film without accompanying deterioration in mechanical physical properties thereof and requiring much capital investment (Patent Document 4).
In addition, a method of producing a chip seal for use in a scroll compressor has been proposed that includes the steps of molding a resin composition containing a fluorine resin as a main component into a columnar or cylindrical molded product, and molding the molded product into a sheet shape state by using a skiving process (Patent Document 5).
Moreover, a method for producing a sintered polytetrafluoroethylene porous sheet has been proposed that includes compression-molding polytetrafluoroethylene powders to form a cylindrical preliminary compression-molded product; hanging horizontally the preliminary compression-molded product by a mandrel; sintering the product to form a sintered porous molded product; and then subjecting the sintered porous molded product to a cutting process (Patent Document 6).